Method of reducing carbon loss from graphite moderator in carbon dioxidecooled nuclear reactor



n 1 .man n May my MW M. if. swbpmmn ET AL EASMB METHOD OF REDUCINGCARBON LOSS FROM GRAPHITE MODERATOH IN CARBC'N DIXDE-COOIJED NUCLEARSEACTOR Filed June 50, 1964 mi ma BMS United States Patent C) 3,320,134METHUD F REDUCING CARBGN LOSS FROM GRAPHITE MODERATOR 1N CARBON DIGXIDE-CQQLED NUCLEAR REACTOR Malcolm Frederick Sheppard, Sale, and HenryChapman Cowen, Y Manchester, England, assignors to United Kingdom AtomicEnergy Authority, London, England Filed .lune 30, 1964, Ser. No. 379,308Claims priority, application Great Britain, July S, 1963, 26,835/63;Feb. 19, 1964, 7,026/64 3 Claims. (Cl. 176-38) This invention relates tonuclear reactors having a graphite moderator and a carbon dioxidecoolant which circulates through the reactor under pressure, the coolantbeing in contact with the moderator.

It is known that in such reactors reaction takes place between thegraphite moderator and the coolant resulting in a loss of carbon fromthe moderator, and that the rate of reaction increases with the level ofirradiation so that in highly rated reactors the loss of carbon from themoderator may be serious. The present invention is particularlyconcerned, therefore, with providing means of inhibiting the reactionand reducing the loss.

Our British patent specification No. 874,487 describes inhibition of thereaction by maintaining a quantity of carbon monoxide in the coolant,the quantity ranging from 0.5 to by volume. The present invention isbased on the discovery that the inhibiting eifect of carbon monoxide isincreased if a paraiiin hydrocarbon is also added to the coolant.

The present invention accordingly resides in one of its aspects in theaddition of a uid parain hydrocarbon to the carbon dioxide coolant of agraphite moderated nuclear reactor.

It is at present -believed that the inhibiting effect of the paraffinhydrocarbons is due to the absorption of the hydrocarbon itself or someradiolytie product such as methyl radicals on the surface of thegraphite moderator, It has however been discovered that theconcentration of .parain hydrocarbons added to the coolant in accordancewith the invention decreases gradually over a period of time and that,whilst the inhibiting effect does not immediately cease, for permanentprotection of the graphite moderator, repeated additions of paratiinhydrocarbons at a controlled rate are considered necessary. Preferablythe additions are continuous so that the preferred concentration ofparaffin hydrocarbons is maintained in the reactor coolant.

The invention accordingly resides in a second aspect, in a carbondioxide coolant for a graphite moderated nnclear reactor, the coolantcontaining at least 50 parts per million by volume of a parainhydrocarbon.

Parain hydrocarbons decompose under irradiation and methane will beformed in reactor coolant to which they are added even if it is notadded specifically. Furthermore, the simplicity of methane makes itespecially useful as an additive. Accordingly, the invention residesfurther in a carbon dioxide coolant containing at least 50 parts permillion by volume of methane.

The inhibiting effect of the hydrocarbons is apparently reduced if watervapour is also present in the coolant and, in general, the less watervapour present the more effective the inhibition by the hydrocarbonspresent and the higher the methane concentration in the coolant thehigher the total quantity of water vapour which may be tolerated. Theinfluence of water vapour on the inhibiting effect is 3,320,134 PatentedMay 16, 1967 ICC more noticeable with increasing concentrations,however, and it is preferable for the water vapour concentration to beless than one half of the concentration of methane in the coolant.

Carbon monoxide will always be present in carbon dioxide coolantdue tobreak-down of thecarbon dioxide under irradiation and may also be addeddeliberately. It has been found, however, that if a parailin hydrocarbonis added to the coolant, in accordance with the invention, the quantityof carbon monoxide required to produce a satisfactory inhibiting effectmay be reduced. Such a reduction is an advantage because it lessens thetendency for carbon deposition to occur in the coolant circuit outsidethe reactor. Examples illustrating the invention `will now be describedand reference will be made to the accompanying FIGURES l and 2 which areboth graphs.

The following examples illustrate the inhibiting effect of the presenceof methane.

Example 1 A specimen of graphite similar to that used for nuclearreactor moderator structures was exposed in a neutron ilux of 3.6 1013to a stream of carbon dioxide. The pressure was approximately 210p.s.i.g., the temperature about 350 C. A similar specimen was thenexposed similarly to a stream of carbon dioxide containing 5.6 v/ocarbon monoxide and 440 p.p.m. by volume of methane. With the methaneand carbon monoxide present the weight loss of the graphite specimen wasreduced by a factor of about twelve.

Example 2 The same kind of specimen as in Example l was exposed underthe same conditions as in Example 1 to a stream of carbon dioxidecontaining 0.44 v/o carbon monoxide, and a similar specimen was exposedto carbon dioxide containing 0.44 v/o carbon monoxide and 500 ppm. byvolume of methane. In this example the weight loss of the graphitespecimen was reduced by a factor of six in the presence of the methane.

Example 3 Under the same conditions as in the previous examples the samekind of specimens were exposed to pure carbon dioxide and to carbondioxide containing 2.2 v/o carbon monoxide and 540 ppm. of methane. Theweight Iloss was reduced in the presence of methane and carbon monoxideby a factor of 15 whereas a similar carbon monoxide concentrationwithout methane gave a factor of reduction of only 1.6.

To illustrate the effect of methane and other hydrocarbons underirradiation pure carbon dioxide and carbon dioxide containingrespectively methane, ethane, propane nbutane and neo-pentane werepassed over carbon-14 labelled `graphite specimens disposed in theneutron ux of an experimental reactor and the carbon-14 activity in theoutlet gas was determined for each mixture as representing the rate ofattack of the graphite specimen. The in hibition factor for each mixturecompared with pure carbon dioxide was then calculated as a ratioactivity in pure CO2 activity in mixture The following results wereobtained: The advantages of maintaining the concentration of water TABLEI Analytical Results, v.p.m. Expt. Sampling Inhibition No. PositionFactor CO H2 H20 CH4 C2110 03H8 I1-C4H1o Ileo-CsI-Iiz 1 Inlet 4, 800 1040 Ounen 4, 800 60 90 10 10 20 1, 400 180 520 20 10 30 1,000 s 420 10010 250 1,100 80 550 20 10 20 Out1et 1, 400 40 450 N.D.=Not determined.f' Y vapour at a low level is shown by the graph of FIGURE 2 whichillustrates the effect on coolant containing 400 20 p.p.m. of methaneand 0.2-0.5 carbon monoxide.

The following results also obtained with the use of carbon-14 labelledgraphite specimens illustrate the effect of varying quantities ofmethane in the coolant:

TABLE II CO, vol. pcrccnt CH4, v.p.m. Hz, v.p.m. H2O, v.p.m. EXPL N0Inhibition Factor In Out In Out In Out In Out 0.48 0.48 190 110 10 60 4090 6.2-8.2 0.42 0. 34 380 190 10 60 20 120 8-10 0. 43 0. 46 430 380 10110 40 180 14. 9-17. 9 (l. 44 0. 47 490 360 10 90 40 180 16-20 0.33 0.38790 500 130 20 250 25-33 1.22 1.14 180 135 10 50 30 35 5.7-7.5 0. 83 O.85 300 190 10 90 50 45 9. 2-10. 8 0. 87 0. 85 300 180 10 110 35 40 8.0-10. 6 1. 67 1.64 85 55 10 60 60 15 3. 0-3. 8 1.82 1. 82 440 340 10 17020 50 13w 1. 7 1. 8 450 400 10 100 40 45 17 5-22. 5 1. 51 1. 47 930 75010 110 45 110 37H19 5. 4 5. 0 430 340 20 100 40 80 10. 7-14. 7

nil 0. 10 540 250 10 130 48 400 10H12 0. 52 0. 56 480 330 950 760 40 2701042 1. 07 1. 15 2, 400 2, 000 550 590 60 420 60-80 *From Table I.

As explained in the paper by Lind and Wright (B.N.E.S. Symposium- TheAdvanced Gas Cooled Reactor-Paper No. the C02-graphite reaction rate isproportional to the energy absorbed by coolant gas in the pores of thegraphite. The extent of the reaction in a nuclear reactor will thusdepend on the amount of coolant gas in the pores of the graphite andthat in turn will depend on the coolant pressure and the open porevolume ofthe graphite. Consequently, the problem of graphite corrosionincreases with coolant pressure. The extent of the reaction also dependson the energy flux. It is therefore necessary to increase the inhibitionfactor as the energy ux of reactors is increased. For example, in areactor of the Magnox type having an energy flux in the region of 3 mw./adjacent tonne it is considered that the presence of 400 v.p.m. ofmethane will give adequate protection but When the energy flux isincreased by using clusters of fuel elements instead of single elements(as in the Windscale Advanced Gas Cooled Reactor which has an energy uxin the region of 20 mw./ adjacent tonne) it is considered desirable thatthe methane concentration should approach 1G00 v.p.m. The effect of thepresence of approximately this quantity of methane in the WindscaleAdvanced Gas Cooled Reactor is shown in the graph of the accompanyingFIGURE 1 which relates to a system also containing Water vapour isformed by breakdown of the added methane under irradiation. Removal ofthe water Vapour is therefore required so long as methane additions arebeirg made to the coolant. A conventional drier may be use Hydrogen isalso formed by breakdown of the methane. If necessary, it may be removedby oxidation which would remove excess carbon monoxide at the same time.

We claim: y

1. A method of reducing the loss of carbon from a graphite moderator ina carbon dioxide-cooled nuclear reactor, the method comprising the stepof introducing a uid paraffin hydrocarbon into the carbon-dioxide coo1ant, to maintain a concentration of at least 50 parts per million byvolume of paran hydrocarbon in the coolant, wherein at least some carbonmonoxide is also present in the coolant.

2. A method according to claim 1 wherein the paraflin hydrocarbon ismethane.

3. A method according to claim 1 wherein the parafhn hydrocarboncontains between two and five carbon atoms.

References Cited by the Examiner` UNITED STATES PATENTS 0.4% carbonmonoxide and between 200 and 300` v.p.m. 2998388 8/1961 L? Lande et al252-67 of water. The results were obtained by insertion of car- 310805110/1963 Lmfistrom v"" 176-38 bon-14 labelled specimens in the reactorand are ex- 35126348 3/1964 Mener. 252-67 pressed as a reduction in theG(-C) value, that is, the 3183166 5/1965 Schweitzer et al 176- 38 numberof carbon atoms removed from the graphite matrix per 100 e.v. of energyabsorbed by the gas in the pores 70 FOREIGN PATENTS of the graphite, theenergy absorption being determined 87A 487 8/1961 Great Britaincalorimetrically. In the absence of methane the G(-C) 2/1963 GreatBritain.

value was 1.

The more water vapour present in the coolant the more methane requiredto produce a given inhibition factor.

L. DEWAYNE RUTLEDGE, Primary Examiner.

1. A METHOD OF REDUCING THE LOSS OF CARBON FROM A GRAPHITE MODERATOR INA CARBON DIOXIDE-COOLED NUCLEAR REACTOR, THE METHOD COMPRISING THE STEPOF INTRODUCING A FLUID PARAFFIN HYDROCARBON INTO THE CARBON-DIOXIDECOOLANT, TO MAINTAIN A CONCENTRATION OF AT LEAST 50 PARTS PER MILLION BYVOLUME OF PARAFFIN HYDROCARBON IN THE COOLANT, WHEREIN AT LEAST SOMECARBON MONOIXDE IS ALSO PRESENT IN THE COOLANT.